Implants have found use in modern medical technology in many different embodiments. They are used, for example, for supporting and/or bracing vessels, hollow organs, and duct systems (endovascular implants, for example stents), for attaching and temporarily fixing tissue implants and tissue transplants, as well as for orthopedic purposes, for example as pins, plates, or screws. Frequently, only a temporary supporting or retaining function is necessary or desired until the healing process is complete or the tissue has stabilized. To avoid complications resulting from the implant permanently residing in the body, either the implants must be surgically removed, or they are composed of a material which is gradually degraded in the body, i.e., is biocorrodable. The number of biocorrodable materials based on polymers or alloys is constantly increasing. Biocorrodable metal alloys of the elements magnesium, iron, and tungsten, among others, are known. One form of implant used particularly often is the stent.
The implantation of vascular supports such as stents, for example, has become established as one of the most effective therapeutic measures in the treatment of vascular diseases. Stents perform a support function in hollow organs of a patient. For this purpose, stents of conventional design have a filigreed support structure made of metallic braces, which are initially in a compressed form for insertion into the body, and are then expanded at the site of application. One of the main fields of application of such stents is to permanently or temporarily widen and keep open vascular constrictions, in particular constrictions (stenoses) of the coronary vessels. In addition, aneurysm stents, for example, used for supporting damaged vascular walls are known.
Stents have a circumferential wall of sufficient load capacity to keep the constricted vessel open to the desired extent, and have a tubular base body through which blood flows through unhindered. The circumferential wall is generally formed by a lattice-like support structure which allows the stent to be inserted in a compressed state, with a small outer diameter, up to the constriction in the particular vessel to be treated, and at that location, for example by use of a balloon catheter, to be expanded until the vessel has the desired enlarged inner diameter. The process of positioning and expanding the stent during the procedure and the subsequent location of the stent in the tissue after the procedure is completed must be monitored by the cardiologist. This may be achieved using imaging methods such as X-ray analysis, for example.
The stent has a base body made of an implant material. Such an implant material is a nonliving material which is used for medical applications and interacts with biological systems. The basic requirement for use of a material as an implant material, which when properly used is in contact with the bodily surroundings, is compatibility with the body (biocompatibility). Biocompatibility is understood to mean the ability of a material to induce an appropriate tissue reaction in a specific application. This includes adaptation of the chemical, physical, biological, and morphological surface characteristics of an implant to the recipient tissue, with the objective of a clinically sought interaction. The biocompatibility of the implant material is also dependent on the time sequence of the reaction of the biosystem which has received the implant. Relatively short-term irritation and inflammation occur which may result in changes in the tissue. Accordingly, biological systems react in various ways, depending on the characteristics of the implant material. The implant materials may be divided into bioactive, bioinert, and degradable/absorbable materials, depending on the reaction of the biosystem.
Implant materials for stents include polymers, metallic materials, and ceramic materials (as a coating, for example). Biocompatible metals and metal alloys for permanent implants contain, for example, stainless steels (316L, for example), cobalt-based alloys (CoCrMo cast alloys, CoCrMo forged alloys, CoCrWNi forged alloys, and CoCrNiMo forged alloys, for example), pure titanium and titanium alloys (CP titanium, TiAl6V4, or TiAl6Nb7, for example), and gold alloys. For biocorrodable stents, the use of magnesium or pure iron, or biocorrodable base alloys of the elements magnesium, iron, zinc, molybdenum, and tungsten, is recommended. Thus, for example, DE 197 31 021 A1 proposes the production of medical implants from a metallic material whose primary component is iron, zinc, or aluminum, or an element from the group of alkali metals or alkaline earth metals. Alloys based on magnesium, iron, and zinc are described as particularly suitable. Secondary components of the alloys may be manganese, cobalt, nickel, chromium, copper, cadmium, lead, tin, thorium, zirconium, silver, gold, palladium, platinum, silicon, calcium, lithium, aluminum, zinc, and iron. Also known from DE 102 53 634 A1 is the use of a biocorrodable magnesium alloy containing >90% magnesium, 3.7-5.5% yttrium, 1.5-4.4% rare earth metals, and the remainder <1%, which is particularly suitable for producing an endoprosthesis, for example in the form of a self-expanding or balloon-expandable stent. The use of biocorrodable metallic materials in implants may result in a considerable reduction in rejection or inflammatory reactions. Such biocorrodable implants and stents frequently also have a coating or cavity filling with a suitable polymer. Also known are stents made of biocorrodable polymers such as polylactide (PLA), poly-L-lactide (PLLA), poly-D-lactide (PDLA), polyglycolide (PGA), polydioxanone, polycaprolactone, polygluconate, polylactic acid-polyethylene oxide copolymers, modified cellulose, collagen, polyhydroxybutyrate (polyhydroxybutyric acid/PHB), polyanhydride, polyphosphoesters, polyamino acids, poly(alpha-hydroxy acid), or related copolymer materials.
One problem with biocorrodable implants, in particular biocorrodable stents, is the difficulty in ensuring a service life which is long enough to achieve the desired medical effect. As a rule, the implant begins to corrode immediately after implantation. Coatings, preferably self-biocorrodable coatings, are known which may retard such a biocorrosion process. However, even for a retarded biocorrosion process the loss of stability or integrity of the implant begins shortly after the implantation, causing the implant to become increasingly unstable over time. Ensuring the stability or integrity of a biocorrodable implant over the desired service life essentially without impairment, and having a biocorrosion process of the implant begin to an appreciable extent only after the end of the desired service life, are currently not possible.
The object of the present invention is to reduce or avoid at least one of the disadvantages of the prior art.